Here's a quick script you can add as a pipeline job to list all environment variables:
node {
    echo(env.getEnvironment().collect({environmentVariable ->  "${environmentVariable.key} = ${environmentVariable.value}"}).join("\n"))
    echo(System.getenv().collect({environmentVariable ->  "${environmentVariable.key} = ${environmentVariable.value}"}).join("\n"))
}
This will list both system and Jenkins variables